Porphyromonas gingivalis (Pg), a Gram negative anaerobe is part of a destructive biofilm found in the oral cavity. There Pg interacts with other bacteria and host cells lining the gingival sulcus. Additionally, the bacteria can be found inside oral epithelial and gingival cells in vivo. We showed previously that Pg hijacks the hosts' clathrin-mediated endocytosis pathway to promote its own uptake. Many microorganisms bind to specific cell surface receptors and are internalized by the same mechanism. Pg is able to bind to several host proteins and many potential receptors have been proposed for Pg. In this application we hypothesize that tissue transglutaminase (TG2) plays a major role in Pg attachment and subsequent internalization. TG2 can be found on the surface of host cells where it forms covalent bonds between free amine and protein- or peptide-bound glutamine residues. TG2 crosslinking activity is Ca2+-dependent and in our preliminary studies we found that Pg association with host cells is much higher in the presence of Ca2+. In addition, TG2-inhibitors block binding of Pg to epithelial cells and there is not interaction of Pg with host cells detectabe in the absence of TG2. Also, TG2 is part of the integrin-network by binding to fibronectin on cell surfaces. In our hypothesized model we propose that TG2 plays a major role in the initial stages of Pg binding to epithelial cells. In Specific Aim 1 we will identify TG2 domains and therefore TG2 activities that are involved in Pg-host associations. In Specific Aim 2 we will analyze the effect of selected TG2 inhibitors on Pg binding to host cells and the effect of inhibitors on host cell viability. TG2 is a major component in several host pathologies and the search for TG2 inhibitors is a very active field. With this application we want to identify safe and stable TG2 inhibitors that might be useful in future animal studies to treat periodontitis.